34 types of monosaccharides with 6 carbon atoms (hexose) exist in total, including 16 types of aldose, 8 types of ketose and 10 types of sugar alcohol. Aldose abundantly existing generally in the natural kingdom includes 6 types of aldose, namely D-glucose, D-galactose, D-mannose, D-ribose, D-xylose and L-arabinose, and other aldose types except those described above are designated as rare sugars. As the ketose, D-fructose exists, so other ketose types are designated as rare sugars. Other ketose types include D-tagatose, D-psicose, D-sorbose, L-fructose, L-psicose, L-tagatose and L-sorbose. Additionally, sugar alcohol is produced by reducing monosaccharides. In the natural kingdom, D-sorbitol exists relatively abundantly but other sugar alcohol types are quantitatively less. Therefore, such sugar alcohol types are designated as rare sugars. Demands toward such various types of rare sugars are emerging, so it is desired to establish a method for stably producing such sugars.
The method for producing such sugars may be performed by approaches of organic chemistry or by biochemical approaches. Regarding an enzymatic sugar conversion process which is carried out as one biochemical approach, patent reference 1 discloses inventions relating to ribitol dehydrogenase oxidizing ribitol in the co-presence of NAD+ to D-ribulose or reducing D-ribulose in the co-presence of NADH to ribitol, and a method for producing the same, as well as a microorganism generating the same, namely Enterobacter aglomerans 221e (FERM BP-4700), and a method for producing ketose or sugar alcohol using the enzyme or a microorganism with the enzyme activity.    Patent reference 1: JP-A-8-56659